The present invention relates to gas turbine engines, and more particularly, to an epicyclic gear system for use in gas turbine engines.
Epicyclic gear trains are complex mechanisms for reducing or increasing the rotational speed between two rotating shafts or rotors. The compactness of planetary or star system gear trains makes them appealing for use in aircraft engines.
The forces and torque transferred through an epicyclic gear train place tremendous stresses on the gear train components, making them susceptible to breakage and wear. For example, the longitudinal axes of an epicyclic gear train's sun gear, star gear, and ring gear are ideally parallel with the longitudinal axis of an external shaft that rotates the sun gear. Unfortunately, many components of epicyclic gear trains, particularly an internal journal bearing within each star gear, are difficult to install and to effectively align. Additionally, because a perfect alignment is rare due to numerous factors (including imbalances in rotating hardware, manufacturing imperfections, and transient flexure of shafts and support frames due to aircraft maneuvers), it is necessary to have a proper amount of lubrication (i.e. an adequate film thickness) between each internal journal bearing and each star gear. Proper lubrication is necessary because misalignment (both parallel and angular) imposes moments and forces on the internal journal bearing which will cause it to contact and wear on the star gear it is disposed in. Unfortunately, to deliver adequate lubrication between each journal bearing and corresponding star gear, many prior art epicyclic gear trains require multiple parts which also require lubrication and are themselves susceptible to wear.